Bay Area Rapid Transit police chief Kenton Rainey, at podium, answers questions about the shooting of a BART officer who was killed by a fellow officer during a news conference Wednesday, Jan. 22, 2014, in Oakland, Calif. Authorities say a San Francisco Bay Area public transit officer who was shot and killed Tuesday by a fellow officer while searching an apartment was looking for a laptop and other stolen items. Rainey declined to disclose any further details of how the shooting of Sgt. Tom Smith occurred. A robbery suspect was in custody. (AP Photo/Eric Risberg)

OAKLAND, Calif. (AP) — The San Francisco Bay Area transit officer who was shot and killed by a fellow officer while they searched an apartment died of a single gunshot wound to the chest, according to autopsy results released Thursday.Bay Area Rapid Transit police Sgt. Tom Smith was wearing a bulletproof vest when he was shot on Tuesday, but the bullet struck an area that was not covered, Alameda County sheriff’s spokesman Sgt. J.D. Nelson said.

Authorities were still trying to determine whether the other officer’s weapon discharged accidentally or if the officer mistook Smith for someone else, Nelson said. Either way, it was an accident, he added.

Smith, 42, was shot while authorities searched a one-bedroom apartment in Dublin for a smartphone, laptop bag and related items stolen during an armed holdup at a train station in Oakland.

He was the first officer killed in the line of duty in the transit agency’s 42-year history. Smith’s older brother, Patrick, said his heart goes out to the family of the fellow officer who accidentally shot and killed his colleague.

“We’re going to mourn the loss of our brother. We’ll never forget him,” Patrick Smith, a field training officer with the Newark, Calif., police told KPIX-TV Wednesday about the death of his brother.

“But there’s someone else in another family that’s affected by this, too, and I feel sorry for them,” said Patrick Smith, whose other brother, Ed Smith, is a deputy with the Alameda County Sheriff’s Department.

Smith’s wife, Kellie Smith, also a BART police officer, told the TV station that she “cherished every moment” she had with her husband, a 23-year BART police veteran who oversaw the department’s detective unit.

The couple has a 6-year-old daughter.

Funeral services for Smith have been scheduled for Jan. 29 at a church in Castro Valley, BART Police Chief Kenton Rainey said. The event will be open to the public.

Smith was shot while searching the apartment of John Henry Lee, 20, a suspect in several robberies on BART property.

BART police said in a statement that the San Leandro Police Department had arrested Lee on Jan. 16 after an automobile burglary and subsequent chase that ended when the suspect lost control of his vehicle and hit a tree.

Lee has pleaded not guilty to second-degree robbery with use of a handgun.

Rainey has said seven BART officers and a sheriff’s deputy were at the scene when the shooting occurred. Five of the BART officers were detectives in plainclothes, including Smith, and the officers knew a suspect already was in custody, he said, noting the officers were following agency policies and training.

Rainey has declined to disclose any further details about how Smith was shot, deferring those questions to the Alameda County Sheriff’s Office, which is investigating the shooting.

Wearing bulletproof vests, the officers began the search by knocking twice on the door of the apartment, Nelson said. The knocks went unanswered, but the door was unlocked, so four BART officers— including Smith— stepped inside with their guns drawn, Nelson added.

Nelson said an officer fired at least one shot that missed Smith’s vest and struck him in the chest. Nelson added that specifics of Smith’s wound will not be available until the pathologist completes his report.

Police have not officially named the officer who shot Smith, but he has been identified as Michael Maes, 50, a 13-year BART police veteran, according to government officials who spoke on condition of anonymity because they were not authorized to speak about the investigation.

Maes’ attorney, David Mastagni, declined Thursday to talk specifically about the incident because of the investigation.

Mastagni said that his client is cooperating with investigators and that Maes “is deeply concerned and his heart and love goes out to the Smith family.”

The Sacramento-based attorney said that he has seen similar unfortunate incidents in his 40-year career representing law enforcement officers. He said it is one of those risks and perils they encounter in protecting society.

“We all should pray for Officer Smith’s family and for Officer Maes’ family as well,” Mastagni said. “It is always heartbreaking when these types of circumstances arise.”

It is not uncommon for an officer to be fatally shot, even if the officer is wearing a bulletproof vest, said Daniel Lawson, a retired San Francisco police captain and the current senior director for Public Safety at the University of San Francisco.

Lawson said Thursday that while there are different types of ballistic vests, most officers on a day-to-day basis wear a Kevlar vest that covers an area just below the Adam’s apple to just below the stomach in the front and most of the back.

“It protects the vital organs, like your heart and lungs,” Lawson said. “But your head and neck are exposed, and the sides are a bit vulnerable as well.”

Lawson said there could be a number of circumstances that caused the BART officer’s shooting, including what angle did the other officer have the gun and their proximity.

“We don’t have all of that information, so it’s hard to conjecture what actually happened,” Lawson said.

About two decades ago, Smith recalled, the Los Angeles Police Department lost an officer when a rifle round got in through the armpit area of her vest.

“There’s no perfect solution,” he said, “depending on what weapon is being used and where the bullet strikes.”

Also Thursday, Rainey told the BART board during its regularly scheduled meeting that two BART uniformed officers wore lapel cameras during the search, but he does not know whether the shooting was captured by the cameras.

All BART officers up to the rank of sergeant are issued the cameras.

Rainey reiterated Thursday that all uniformed officers are required to wear the cameras, but the devices are optional for plainclothes detectives.

“Obviously, something went wrong, and we will not hide from the truth,” Rainey told BART board members.